Tests for diagnosing Thyroid disease

Blood tests. Blood can be taken at any time as the thyroid hormones and pituitary hormone do not vary greatly throughout the day (circadian variations) and do not change with food intake.  

To determine whether thyroid function is normal, the pituitary hormone thyrotropin (TSH) and active thyroid hormones are measured. Thyroid hormones circulate in the blood partly bound to proteins such as albumin or thyroxine-binding globulin.  For analytical purposes, it is useful to know the portion of thyroid hormones not bound to the proteins: free thyroxine (T4L) and free triiodothyronine (T3L) since, in certain physiological situations, including pregnancy and contraceptive use, the level of proteins in the blood can change, but without altering thyroid function.  

Cervical ultrasound. This is the reference technique for imaging thyroid diseases, especially for the morphological study of thyroid nodules. It is a technique that uses ultrasound (high-frequency sound waves). It is painless and safe, and has no contraindications, it does not require the interruption of any kind of medication, it does not involve irradiation, and it provides excellent information on the image of the thyroid gland (it detects nodules of 1-3 mm) in addition to cervical lymph nodes.  

It is particularly used to study and characterise thyroid nodules. The vast majority of nodules (90%) are benign. Ultrasounds make it possible to detect nodules suspected of being malignant and which require a cytological examination according to a risk classification system.  

The ultrasound technique is also used to guide diagnostic fine needle aspiration (FNA, cytological examination) and in the percutaneous treatment of thyroid nodules, e.g., emptying thyroid nodules that have a liquid content (cysts), introducing substances (alcohol) that sclerose (harden) the nodule, or reducing the size of symptomatic thyroid nodules through the skin.  

Ultrasound can also be used to monitor thyroid nodules and goitres, but it does not provide much information when the thyroid is diffusely enlarged and is not useful for evaluating thyroid function. 

Thyroid scan. This test is performed in nuclear medicine and allows the thyroid function to be imaged. It is particularly useful for studying hyperthyroidism.  

In the week prior to the test, excess iodine from food sources (iodised salt, seaweed, shellfish, etc.) and nutritional supplements containing iodine should be avoided. If a CT scan using iodinated contrast has been performed, the test should be postponed for at least one month and treatment with thyroid-slowing drugs should be discontinued to allow the tracer to accumulate in the thyroid during the test. The medical team should be informed, but amiodarone should not be discontinued. 

This imaging test shows how well the thyroid is functioning, whether it is inflamed and whether the whole thyroid is functioning homogeneously or heterogeneously, where that part of it is working too little or too much. It is useful in cases of hyperthyroidism, but does not provide information on hypothyroidism or normal thyroid function.  

It is based on the fact that the thyroid picks up iodine or iodine-like substances from the blood to make thyroid hormones. An isotope of iodine (I-123) or technetium pertechnetate (Tc-99) is administered and then an x-ray is taken (with the help of a device called a gamma camera) to show which areas of the thyroid have taken up the isotope. It is a short, non-claustrophobic test that uses irradiation.  

Tc-99 is a minimally irradiated isotope that does not require special precautions. It was the first kind of scan to be used in thyroid studies but, due to the development of ultrasound, its use has decreased in recent decades. 

It is used to study single or multiple thyroid nodules (goitre) in the context of hyperthyroidism and to determine the cause. It is also used to treat hyperthyroidism: in Graves-Basedow disease, in autonomous nodules due to the affinity of Iodine-131 for treating hyperthyroidism, and for reducing the size of the nodule.

Ultrasound-guided fine needle aspiration (FNA). This test is used to take a cytological sample of thyroid cells by puncturing the thyroid and then studying the cells under the microscope to determine whether they are malignant or benign. The patient does not need to fast or stop taking any medication prior to the test, except for antiplatelet and anticoagulant drugs as these can increase the risk of bleeding, and so they are replaced with heparin. Usually, the puncture causes no complications that could impede normal activities. Even so, it is recommended that the patient avoids any physical exercise that involves significant contraction of the cervical muscles on the same day.  

It is performed after cleaning the neck, without local anaesthesia. A fine needle is inserted through the skin and the thyroid nodule cells are aspirated, or sucked out, for less than one minute. It may be necessary to insert the needle into different parts of the nodule, which could involve 2 to 3 punctures in each procedure. After the puncture, there may be slight local pain that improves after taking paracetamol or conventional anti-inflammatory drugs. 

Depending on the characteristics of the cells analysed, the following categories are established according to the risk of malignancy: 

• Benign (up to 70% of biopsies). The risk of malignancy in this group is usually very low, less than 3%. If these nodules are asymptomatic and thyroid function is normal, the patient is usually monitored with regular ultrasound scans and laboratory tests.  
• Malignant (3-7% of biopsies). There is a 97-99% chance of these being cancerous. The most common type of thyroid cancer is papillary thyroid cancer. These nodules require surgery (thyroidectomy). 
• Suspected malignancy (variable frequency). This means that the pathologist cannot make a definitive diagnosis of malignancy, but the risk of cancer is 60-75%. The usual treatment is surgery. 
• Atypia of uncertain significance or follicular lesion of undetermined significance. The risk of malignancy is 5-15%. The cells have special characteristics that make it impossible to know whether they are benign or malignant. The puncture must be repeated and/or molecular and genetic tests must be carried out. 
• Follicular neoplasm. This category has a malignancy risk of 15-30%. The appearance of the cells makes it difficult to tell whether this nodule is a benign condition, called follicular adenoma, or a malignant nodule, known as follicular carcinoma. In general, the affected half of the thyroid is removed surgically, for diagnosis and treatment. 
• Non-diagnostic (20% of biopsies). This category includes samples where there are not enough cells to make a diagnosis. Despite a successful puncture, sometimes the sample does not have enough thyroid cells to enable a correct diagnosis, or there is too much blood or only fluid from the cyst. In these cases, the puncture is repeated according to the assessment of the medical team. If the cytology is again non-diagnostic, surgery is considered based on the clinical history and ultrasound details of the nodule.  

Cervical and thoracic computed tomography (CT). This is less specific for thyroid diseases. It is used to determine the limits of the thyroid in large goitres or when the thyroid is suspected to be located lower in the chest behind the sternum. It can also be used to see if the thyroid has grown posteriorly, behind the oesophagus, or inferiorly, behind the sternum. It is a short, non-claustrophobic test that uses irradiation. Contrast is not usually used, so there are no adverse reactions. 

Magnetic resonance imaging. It is not usually used for the study of benign thyroid nodules or goitres. It is particularly useful in the case of thyroid cancer to assess infiltration of local structures, as it has a better resolution than CT images.